A sense amplifier (SA) is used to sense the difference between two signals at two inputs of the sense amplifier, and amplify the difference or, commonly stated, amplify the differential signal across the inputs. To sense correctly, a sense margin, which is the minimum difference between the two signals, or the minimum differential signal, is required.
In many situations, process variation causes two different threshold voltages to be present in two transistors that are supposed to be the same (e.g., two NMOS transistors of a cross latch of an amplifier). In that situation, the sense amplifier is said to have an offset at the two inputs caused by the mismatch of the two transistors. As a result, the sense amplifier favors reading one logic state versus another logic state, instead of neutrally providing the sensed data. For example, some sense amplifiers, when sensing the differential signal across two inputs, tend to favor a high logic level, (e.g., a High) while some other sense amplifiers tend to favor a low logic level (e.g., a Low). A sense amplifier tending to favor a High quickly provides the High as the read data at the output when the actual High data is sensed, but slowly provides a Low as the read data at the output when the actual Low data is sensed. Similarly, a sense amplifier tending to favor a Low quickly provides the Low when the actual Low data is sensed, but slowly provides a High when the actual High data is sensed.
In effect, because of the process variation, the mismatch manifests as an additional offset, which causes the sense amplifier to require a larger sense margin to correctly sense the data. In applications where one sense margin is used for many amplifiers, a larger value for the sense margin is utilized so that a very high percentage, or all, of the sense amplifiers function correctly. As the large sense margin is used, the time required for the differential signal to develop increases, making the overall memory access speed slower.
Like reference symbols in the various drawings indicate like elements.